You can't stop all of it. Some things help though:
If you post to newsgroups, make sure the return address you use
there has some extra letters added after the @, if it's even partially
correct.
If a spammer uses the same return address consistently, click on
such a message, then Message, then Create rule from message,
then Delete it, then OK.
If the spammer puts something in the subject consistently, and
no messages you want to read put the same thing there, click
on such a message, then Create rule from message, then the
box before When the From line contains people, then the box
before Where the Subject line contains specific words, then
Delete it, then the underlined contains specific words, then
type what the spammer put there that you don't want to see
again, then click on Add and then OK and then OK again.
